FIG. 1 shows an example of a conventional four transistor (4T) active pixel used in a Complementary Semiconductor Oxide Semiconductor (CMOS) image sensor. The pixel comprises a pinned photodiode, a transfer gate TX and a sense node A. A buffer amplifier, such as source follower M1, connects to the sense node A. A selection transistor M2 connects the output of the buffer amplifier M1 to a column bus when the pixel is read. A reset transistor M3 also connects to the sense node A in order to set the voltage on the sense node A at a certain level before charge transfer. The transfer transistor TX transfers substantially all charges from the pinned diode to the sense node after reset of the sense node. The pixel is read before and after this charge transfer and the difference between these two samples will be output by the image sensor as the pixel value. This differential operation is called correlated double sampling (CDS).
The conversion gain of the pixel is determined by the capacitance of the sense node A. Typically, conversion gain is expressed in microvolts per electron (μV/e−) and specifies the signal change that is measured when an electron is added to the sense node. The capacitance of the sense node is composed of several parts. Part of this capacitance is a parasitic capacitance such as the junction capacitance of the transfer gate and reset drain junctions, the sidewall capacitance of these junctions, the gate-drain overlap capacitances of the transfer gate and the reset transistor, routing capacitance of the contact of the sense node to the input of the buffer amplifier. In some cases, there is some intentional capacitance added through capacitor devices, such as metal-metal or poly-poly capacitor plates, or metal-metal fringe capacitors or other capacitor devices as typically used in CMOS integrated circuits.
CMOS pixels may be operated in a wide variety of light conditions and signal levels. At high light levels and/or long exposure times, the amount of accumulated charges can be large. This has to be converted into a voltage by a large capacitor. At low light levels or short exposure times, the amount of charges collected can be small. The conversion gain has to be chosen for the expected maximum signal that is expected. If that is large, the conversion gain is low. Small signals, such as dark areas in an image, are converted into a very small voltage.
The charge capacity of the pixel, expressed in electrons, is the maximum signal that the pixel can contain. It can be limited by the photodiode, or by clipping in the readout path or on the sense node. Temporal read noise refers to the temporal variation of a pixel when successive readings are done of the same signal. This is specified in dark conditions, and expressed in electrons. The ratio between the charge capacity and the read noise is the dynamic range. For a higher dynamic range, the noise must be reduced. The most significant noise contribution of a CMOS pixel comes typically from the in-pixel buffer amplifier. In most cases a source follower is used, and the read noise of the pixel, after correlated double sampling, is determined by the thermal and low frequency (1/f and random telegraph signal) noise of the pixel source follower. To readout small charge packets, it is of interest to increase the conversion gain as much as possible in order to reduce the contribution of the noise to the signal. However, if the expected amount of charge is large, the conversion gain will be designed at a lower value, which results in higher read noise.
Typical camera systems operate the image sensor under a variety of gain settings, which are adjusted to the exposure settings and light level, and by the preference of the user of the camera. Under low light conditions, a higher gain is applied.
Pixels with selectable conversion gain have been proposed in several patents and patent applications. U.S. Pat. No. 7,075,049 shows a structure where an additional switch is connected to the sense node. This switch connects an extra capacitor to the sense node. When the switch is on, the pixel has a low conversion gain. This setting is used under high exposure values. When the switch is off, the pixel has a high conversion gain. This setting is used under low exposure values and offers lower read noise of the pixel. U.S. Pat. No. 7,432,540 shows a similar principle, describing a particular implementation of the switch and the capacitor. U.S. Pat. No. 7,705,900 shows a configuration of pixels where sense nodes of neighbour pixels can be connected together by additional switches. These switches allow summing the signal of two neighbour pixels. But these switches also allow increasing the sense node capacitance by connecting the sense nodes of one or more adjacent pixels to the sense node of the pixel that is readout. This allows modification of the sense node capacitance according to the desired conversion gain. FIG. 2 shows an pixel similar as described in U.S. Pat. No. 7,075,049 where an additional transistor M5 connects an additional capacitor Cl to the sense node A when the HDR control signal is high. When HDR is high, the conversion gain is low and proportional to 1/(Cs+Cl). When HDR is low, the conversion gain is high and proportional to 1/Cl.
All the above embodiments make use of an additional transistor (switch) connected to the sense node A. Besides the drain of the transfer gate and the source of the reset transistor, and the input of the source follower, an additional transistor is connected to the sense node A. Connecting an additional transistor increases the parasitic capacitance of the sense node. This increases the capacitance of the sense node, and results in a lower maximum conversion gain for the high gain case. This limits the minimum temporal read noise that can be achieved with such pixel.
US 2008/0237446 A1 describes an image sensor which is operable in a high sensitivity mode and a low sensitivity mode.